This invention relates to a new class of microporous adsorbents for gas separation and purification. For example, the novel adsorbents of the present invention may be used in pressure swing adsorption (PSA) or thermal swing adsorption (TSA) PPU (Pre-purification Unit) for removal of CO2, H2O, N2O, and oil vapor from air streams prior to cryogenic air distillation, hydrocarbons/CO2 separations, and syngas separations. In particular, the present invention relates to novel microporous metallo-organic adsorbents having pore sizes and pore volumes that are appropriate for gas separation and purification.
Presently, numerous microporous materials that are mainly based on zeolites, other zeo-type inorganic solids, various types of activated carbon, such as carbon molecular sieves and super-activated carbons, are being used as solid adsorbents for gas separation and purification. Zeolites are porous crystalline aluminosilicates, whose framework is constructed by SiO4 and AIO4 tetrahedra, which are joined together in various regular arrangements through shared oxygen atoms, to form an open crystal lattice containing uniform pores. Each aluminum atom within that lattice introduces one negative charge on zeolite framework which must be balanced by a positive charge of an exchangeable cation. After activation of zeolites, the exchangeable cations are located, in most cases, at preferred extra-framework sites within the voids formed by the lattice. These cations play a significant role in determining the adsorption properties of the particular zeolites. Moreover, zeolites have high chemical and thermal stability owing to their inorganic nature of the framework. Unlike zeolites, the structure of activated carbons mainly consists of elementary microcrystallites of graphite. These microcrystallites are stacked together in random orientation. The micropores are formed by the spaces between those microcrystallites, the diameter of which range from xcx9c3xc3x85 to xcx9c20xc3x85. In addition, there may exist mesopores (20 to 500xc3x85) and macropores ( greater than 500xc3x85), as well. Therefore, activated carbon adsorbents generally show very little selectivity over molecules with different sizes. However, due to the nonpolar surface of carbon, an activated carbon tends to be hydrophobic and organophilic. With respect to carbon molecular sieves, their structure is similar to that of activated carbon in most general terms, but they have a very narrow distribution of micropore sizes ranging from about 3 to 9xc3x85, for the various types, and thus they behave as molecular sieves.
Clearly, all of these materials, zeolites, activated carbon, and carbon molecular sieves, combine microporosity with high chemical and thermal stability, which are essential for gas separation and purification. Although it is difficult to design and build the microporous structures of these sieve materials with specific pore sizes in a systematic way, a wealth of structures is known by now. Variation of chemical formulation and functionality, however, meets limits due to complexity of phenomena and costs involved. The present invention is directed to novel microporous adsorbents which can be designed with specific pore sizes in a systematic way.
It is the primary object of the present invention to provide a novel class of microporous adsorbents for gas separation and purification.
It is another object of the present invention to provide practical applications of the microporous adsorbents of the present invention in gas separation and purification processes.
To achieve the foregoing objects and advantages and in accordance with the purposes of the invention as embodied and broadly described herein, a new class of microporous adsorbents comprises a metallo-organic polymer, which is characterized by a formula set forth below:
[R(L)n]mMn,
wherein
R is an organic spacer;
L is a ligation group substituted on an organic spacer, e.g., carboxylate group, xe2x80x94C(xe2x95x90O)Oxe2x88x92; dithiocarboxylate group, xe2x80x94C(xe2x95x90S)Sxe2x88x92; and xcex2-diketonate group, xe2x80x94C(xe2x95x90O)C(Rxe2x80x2)xe2x95x90C(xe2x80x94Oxe2x88x92)xe2x80x94, Rxe2x80x2xe2x95x90H; or an aliphatic or aromatic group;
n denotes the number of ligation group, n greater than 2;
M denotes transition metal or rare earth metal, excluding Co, Cu, Zn, Tb when the organic spacer is a benzene ring, the ligation group is carboxylate, and n equals 2 or 3; and
m denotes the oxidation state of transition metal.
Structural modification of the new class of microporous adsorbents can also be made to further enhance desirable functionalities. For example, the channel linings can be chemically functionalized using organic bases, or they can be doped with inorganic salts such as lithium, silver, copper(I/II) salts to create specific adsorption sites that are necessary for gas separation and purification.
Unlike zeolites, other microporous zeo-type materials and carbons, the new adsorbent materials provide unique surfaces which are comprised of carbon, hydrogen, oxygen and nitrogen atoms, and framework metal sites. Owing to their tunable organic and metallic parts of the microporous structures, they have advantages over presently utilized adsorbents in engineering of specific pore sizes and sorption sites in a systematic way. Therefore, along with zeolites, carbon molecular sieves and other carbonaceous adsorbents, this new class of adsorbents expand material selections for industrial gas separation applications.